Objects, such as printed circuit boards (PCBs), liquid crystal glass, plates, films or other substrates that are generally substantially planar and rectangular in shape, occasionally develop defects such as chips, cracks or bumps along the object edges during fabrication or later. Some of these defects are minor and can be ignored because, for example, they are relatively small and will not affect the operation of the objects. However, larger defects that have the potential to adversely affect the proper operation or use of the object need to be identified so that appropriate action can be taken. For example, the object might be discarded. It may therefore be advisable to subject the edges of such objects to an automated inspection to check for defects.
One conventional automated system and method of detecting defects on the edge of an object uses a single electronic edge measurement sensor or position sensor. This arrangement is illustrated in FIG. 1A where an object such as a printed circuit board 1 is inspected by a single position sensor 10 that inspects the edge of board 1. Sensor 10 measures a value d1 that is the distance of the edge of the board detected by sensor 10 from a “0” point of the sensor, which is generally set to be at the center of the sensor. If there is a chip 5 along the board edge, it is determined whether d1 is less than ε, where ε is a predetermined threshold value that is programmed by a user and corresponds to the maximum acceptable deviation of the board edge from the edge position that provides a “0” sensor reading. If |d1|≤ε, then a determination is made that the board is “normal.” However, if |d1|>ε, then a determination is made that there is a defect. This manner of edge inspection may be adequate to detect a defective edge where the width of board 1 is fixed at a value W±ε along the length of the board when there is no shifting/misalignment of the board in a sideways direction, tilting of the board, or fluctuation of board width (i.e., a change in width from one board to the next). If there is shifting, tilting or fluctuation, inspection with a single sensor is adequate where the amount of shifting, tilting, or fluctuation of board width is known and can be taken into account. However, even when a fluctuation λ of the board width is taken into account, the edge detection performance will deteriorate because the threshold for generating a finding that a defect is present will increase from ε to ε+λ (i.e., |d1|>ε+λ).
FIG. 1B illustrates the consequence of allowing for a deviation of ε in board width when inspecting board edges. The deviation of ε is permitted to be in either direction, + or − from the “0” point of the sensor 10. Therefore, the deviation allows for the board width to possibly be as wide as W+ε or as narrow as W−ε. Where the board width is W+ε, a chip 5 in the board edge can be as wide as 2ε before the board will be determined to be defective.
FIG. 1C shows graphically the measured value that is detected by sensor 10 over time when there is a chip 5 in the left edge of board 1 and board 1 is moved in the direction shown by arrow 12 of FIG. 1A. As FIG. 1C shows, when board 1 is found to have one or more chips that exceed a threshold ε (indicated by symbol 15), then board 1 is found to be defective.
FIGS. 1D and 1E illustrate the consequences of inspecting a board 11 that has no defective edges with a single sensor 10 where board 11 has been shifted to the right relative to sensor 10 by an amount Δ. The shifting of board 11 relative to sensor 10 is indicated by the skewed arrow in FIG. 1D. As shown in FIG. 1E, the shifting can lead to a “false positive” edge detection measurement that exceeds a defect threshold ε as indicated by symbol 15 even though there is no defect on the edges. As a result, a non-defective board will be rejected as defective, which is undesirable.
FIG. 2A shows another conventional manner of automated defect detection of object edges for defects such as chips, cracks or bumps using two aligned electronic sensors 10, 20, one for each of the opposite sides of an object such as printed circuit board 1 to be inspected. The distance between sensors 10, 20 is W. A movement of the left board edge to the left of sensor 10 or a movement of the right board edge to the right of sensor 20 (as shown) is measured as a negative movement, while a movement of a respective edge in the opposite direction is measured as a positive movement. FIG. 2A shows board 1 with a chip 5 along the left edge. The width of board 1 is approximately W±ε along the length of the board and board 1 is shifted to the right from the center of sensors 10, 20 by a distance Δ. A single combined measurement of both sensors 10, 20 is used to determine whether the board is chipped. As the board width W±ε is fixed and the distance between the “0” point of the two sensors 10, 20 is set approximately equal to W, then status of the board can be judged as follows:
Normal: |d1+d2|≤ε
Defective: |d1+d2|>ε
Any shift value Δ is canceled out in calculating |d1+d2|. If the board width is as large as W+ε, a chip in the board edge can be as wide as 2ε, before it is detected as defective.
FIG. 2B shows a board 21 that has no defective edges but has a width that fluctuates by λ from the width of another board to be inspected. This fluctuation can affect performance. If the fluctuation is known, it must be taken into account but the edge detection performance will deteriorate because the threshold for a chip to generating a finding that a defect is present will increase from ε to ε+λ. If the fluctuation is not taken into account, then the fluctuation in width may be detected as a defect even though the board 21 is not defective.
FIG. 2C shows graphically the measured value of each of sensors 10, 20 over time when the board width fluctuates as shown in FIG. 2B. In FIG. 2C, sensor 10 is referred to as A and sensor 20 is referred to as B. FIG. 2D shows the result of adding A+B and comparing this sum to a threshold that is specified for the board. Where the width of board 21 fluctuates too greatly from the normal width, the board is improperly identified as defective by virtue of a “false positive” reading.
Thus, as noted above, one drawback of conventional defect detection techniques is that a possible shift of the board must be accounted for or a false positive defect detection may result. Moreover, a width fluctuation of a board can lead to a false positive finding of a nonexistent defect when only one or two sensors are used for edge inspection and the fluctuation is not specifically addressed. It would therefore be advantageous to have a more flexible system that accounts for possible board shifts/misalignments or fluctuations in the width of an object and provides correct indications of whether or not a board has defective edges.